Characteristic Mode Analysis of a Metallic Ring Antenna

The normalised surface current distributions and far field patterns of the first four eigencurrents of a metallic ring antenna based on the antenna described in [1] are calculated in FEKO.

The first four modal currents and associated far fields of a ring antenna with four slots bridged with excitation ports.

Mode J₃ of the ring is a non-resonant mode with inductive behaviour in the frequency band of interest.  A mode resonates when the eigenvalue is zero and the characteristic angle is 180 degrees.  A positive eigenvalue indicates a mode that is storing magnetic energy and a negative eigenvalue one that is storing electric energy [1].  The characteristic angle of mode J₃ is below 180 degrees and its eigenvalue is positive across the frequency band.

Characteristic angle plot for the ring antenna showing resonating and non-resonating modes are present.

Antonino-Daviu et al. show in [1] how the antenna can be modified to turn mode J₃ into a radiating mode.  Capacitive loading is added to move the modal resonances closer in frequency thereby resulting in a MIMO antenna.

[1] E. Antonino-Daviu, M. Cabedo-Fabres, M. Gallo, M. Ferrando-Bataller, and M. Bozzetti, “Design of a multimode MIMO antenna using characteristic modes”, EuCAP 2009.